The present invention relates to apparatus for preheating mold resin suitable for incorporation in molding apparatus used for manufacture of semiconductor devices such as central processing units or microprocessors of computer systems or memory devices.
An example of semiconductor molding apparatus is formed, as shown in FIG. 1, to mold, by means of molding resin 2, semiconductor pellets (not shown) mounted on. lead frames 1 to provide protection for the pellets.
As illustrated in FIG. 2, the semiconductor molding apparatus may comprise an upper mold die 3 and a lower mold die 4 forming a pair with each other, and a lead frame loader 5 movable in and out of between the upper and the lower molds dies 3 and 4 to carry a lead frame in and out, and a tablet loader 7 for supplying pots 6 (FIG. 3) provided in the lower mold die 4, with tablets, each of which is formed of molding resin and has a shape of rod or the like.
FIG. 3 is a plan view of the lower mold die 4. As illustrated, a plurality of cavities 8 are provided on the surface of the lower mold die 4. In the illustrated example, the cavities are arranged in two rows (horizontal alignments as viewed in FIG. 3) and four columns (vertical alignments as viewed in FIG. 3). Provided between the cavities 8 of the same columns are pots 6 for charging the resin. The upper mold die 3 has cavities (not shown) provided on the lower surface thereof at positions conforming to the cavities 8 of the lower mold die 4. Gates 9 are provided to transfer resin in a fluid state from the respective pots 6 to the adjacent cavities 8.
In operation, the lower mold die 4 is lowered, and a lead frame loader 5 is inserted between the upper and the lower mold dies 3 and 4 to carry a lead frame 1 into position in the lower mold die 4. The lead frame loader 5 is then withdrawn and a tablet loader 7 carries tablets in and drops them into the respective pots 6. By then, the lower mold die 4 is heated to a temperature (e.g., 180.degree. C.) sufficient to melt the tablets into a fluid state. The tablet loader 7 is then withdrawn, and the lower mold die 4 is elevated to mate with the upper mold die 3. Cavities of the desired shape corresponding to the shape of the resin portion 2 of FIG. 1 are formed at the mating surfaces of the upper and the lower mold dies 3 and 4 by the cavities 8 of the lower mold die 4 and the corresponding cavities of the upper mold die 3. Plungers (not shown) disposed in the pots 6 are then moved upward to push up the fluid resin so that the fluid resin is injected through the gates 9 into the cavities. After the injection is completed the resin is set or cured in a short time because of its thermo-setting property. The lower mold die 4 is thereafter lowered (being separated from the upper mold die) and the resin-molded semiconductor devices as shown in FIG. 1 are taken out. Thus the molding process is completed.
With the above-described molding apparatus, it takes several seconds or ten and several seconds until the tablets are heated to a melting temperature (170.degree.-190.degree. C.) after being introduced in the pots 6. The time necessary for the heating varies depending on the dimensions of the tablet. The time needed for the resin to be cured after it is introduced into the pots should be preferably shorter from a viewpoint of productivity, i.e., to reduce the molding cycle time. The concentration of cure accelerator in the resin may be increased to reduce the time necessary for the curing, but this will degrade the water resistance of the product. The time for heating the resin tablets should be sufficient. If it is insufficient, longer formation time may be necessary or wire deformation may occur. These are particularly problematical in the manufacture of a large-scale device such as a microprocessor. If the heating temperature is raised, the heating time can be reduced, but the formation conditions become unstable and some cavities may not be completely filled with the resin.
To solve the above-described problems, tablets may be preheated before they are introduced in the pots to such a degree that they are in a semi-melted state. The preheating can be effected by means of high frequency (e.g., 70 MHz, or 570 MHz) heating. But this method has a limitation in that the heating electrodes must be scaled to the small size of the tablets and must be positioned in the proximity of the respective tablets to attain uniform pre- heating.